1. Field of the Invention
The present invention relates to a hard disk drive.
2. Description of the Related Art
In recent years, an information recording/reproducing apparatus employing a hard disk drive (hereinafter, referred to as “HDD”) is being widely used in a portable music player, a mobile phone, or the like. The information recording/reproducing apparatus is desired to be further downsized. Accordingly, the HDD to be mounted to the information recording/reproducing apparatus is also desired to be downsized and thinned.
In view of the above, an information recording medium (hereinafter, referred to as “magnetic disk”) used in the HDD has a higher recording density, and thus attaining a larger capacity. Along with the attainment, a small (small in diameter) and thin magnetic disk is being developed.
The magnetic disk is mounted onto a disk mounting surface provided to a rotor hub of a spindle motor for driving the HDD. An upper surface of the magnetic disk is pressurized by a disk clamper, and the magnetic disk is thus fixed. The magnetic disk rotates at high speed together with the rotor hub of the spindle motor in recording/reproducing the information. Since the magnetic disk has the larger capacity, in order to accurately perform the information recording/reproducing, requirements to the magnetic disk, such as a rotational speed and a rotation precision, are becoming severer.
The rotor hub of the spindle motor as described above has employed a ferritic stainless steel in consideration of the following advantages.                (1) A ferritic stainless steel has good machinability, so a dimensional precision is readily secured.        (2) A ferritic stainless steel has a coefficient of linear expansion similar to that of a material (glass) of the magnetic disk.        (3) A ferritic stainless steel has corrosion resistance to a certain extent.        
Further, since the magnetic disk has the larger capacity, in order to accurately perform the information recording/reproducing, deformation of the magnetic disk mounted on the disk mounting surface is required to be minimized. Accordingly, flatness of the disk mounting surface of the rotor hub, which is brought into contact with the magnetic disk, should have a flatness close to that of the disk.
As a related art, to solve the problems described above, there is proposed a technique in JP 2006-155864 A, in which the disk mounting surface of the rotor hub is finished while leaving spiral cutting marks. In this case, surface roughness is increased due to the cutting marks. However, because the magnetic disk is supported only by the projecting portions of the cutting marks, so distortion of the magnetic disk decreases. Note that examples of a material of the rotor hub include, in addition to a ferritic free-machinable stainless steel, a martensite-based stainless steel, an austenitic stainless steel, aluminum, alloy thereof, brass, copper alloy, various kinds of steel, and machinable materials having proper hardness.
Further, a ferritic stainless steel has good machinability to a certain extent. However, it is difficult to control the disk mounting surface so as to have the flatness of 1 μm or less. That is, the flatness (processing accuracy) of a machine-processed surface suffers from affects of vibration generated by load of a processing machine (e.g., lathe) with respect to the spindle and vibration generated by a cutting tool. Thus, in order to improve the flatness, selection of a material having good machinability is required. Accordingly, the machinability of the ferritic stainless steel is poor in order to secure the flatness of 1 μm or less for the disk mounting surface.
In order to solve the problems concerning the flatness to reduce the deformation of the magnetic disk, for example, JP 2005-228443 A discloses a structure in which a spacer is interposed below the disk mounting surface.
Along with attainment of the magnetic disk having a larger capacity and a thinner structure, in the related rotor hub made of a ferritic stainless steel (product name: SF20T, manufactured by Shimomura Tokushu Seiko Co., Ltd.), affects of various additives added to improve the machinability thereof does not become neglectable. Examples of the additives for improving the machinability include sulfur (S), tellurium (Te), selenium (Se), lead (Pb), and manganese (Mn).
To be specific, the inventors of the present invention have found that, when component particles of the additive fall from the disk mounting surface of the rotor hub, there are formed recessed portions of minute pores caused by the fallen particles on the disk mounting surface. Such failings of the component particles are particularly caused by tellurium among the additives for improving the machinability.
FIG. 7 is a photograph showing a cross section of a rotor hub 12′ while the cross section is taken at 400-magnification. In the cross section, there exist a large number of elongated component particles P.
When the rotor hub 12′ is machine-processed to form the disk mounting surface 15′, the component particles P present in the machined surface and the vicinity thereof and exposed to the disk mounting surface 15′ sometimes fall. Accordingly, at portions of the disk mounting surface 15′ from which the component particles P have fallen, there are formed recessed portions (spaces) of the fallen-particle pores.
FIG. 8 is an enlarged photograph showing a vicinity of an outer-circumferential-side edge portion of the disk mounting surface 15′, having a ring shape in a plan view. There exist recessed portions of relatively-large fallen-particle pore D having a length DL in a circumferential direction of more than 10 μm. It has been found that, when such recessed portions exist on the disk mounting surface, the thin magnetic disk which is fixed by the application of a pressing force of the disk clamper, distorts at the recessed portions, causing minute deformation.
In addition, it has been found that, with regard to projecting portions of cutting marks on the disk mounting surface 15′, when there exist pores (having a length DB in a diameter direction) on at least both a projecting portion located in an outermost circumference and a projecting portion located adjacent thereto inwardly, even though the length DL of the pores in the circumferential direction is less than 10 μm, the pores largely affect on the minute deformation of the magnetic disk.
In other words, it has been found that, in a high-density magnetic disk, when there exists on the disk mounting surface recessed portions of minute pores caused by fallen particles of an additive, the minute deformation caused by the fallen-particle pores hinders accurate recording/reproducing of information.
Note that the disk mounting surface 15′ has a minute width, e.g., about 0.25 mm, and only supports an inner-circumferential-side edge portion of the ring-shaped magnetic disk.
FIG. 9 shows a state where a magnetic disk which is minutely deformed on the disk mounting surface 15′ having the fallen-particle pore D of FIG. 8. Referring to FIG. 9, there generates deformation in which the inner-circumferential-side edge portion of the magnetic disk supported by the disk mounting surface 15′ is recessed, and an outer-circumferential-side edge portion is projected. In the example of FIG. 9, there generates a deformation in which the largest recess (−1.83343 μm) occurs in the vicinity of a portion where the fallen-particle pore D exists, and the largest protrusion (+1.23012 μm) generates in an outer circumferential portion of a circle substantially having the same radius. A maximum deformation amount, which is obtained by summing the largest recess and the largest protrusion, is 3 μm or more, and is a relatively large minute deformation.
With the above-mentioned background, there has been desired to develop a hard disk drive including a flat disk mounting surface which does not hinder accurate recording/reproducing of information even in employing the high-density magnetic disk, i.e., a flat disk mounting surface having no recessed portion which may cause relatively large minute deformation hindering accurate recording/reproducing.